Selective Gas Adsorption in Permanently Microporous Coordination Cages with Exposed Metal Sites

Abstract

Porous coordination cages (PCCs), molecular analogs of metal–organic frameworks, offer modular platforms for studying the adsorption properties of small molecules, with coordinatively unsaturated metal centers playing a pivotal role in tuning these behaviors. In this work, we present the synthesis, activation, and detailed gas adsorption studies of second-row transition metal-based M₂₄L₂₄ cuboctahedral cages, specifically Mo₂₄(bdc)₂₄, Rh₂₄(bdc)₂₄, and [Ru₂₄(bdc)₂₄]Cl₁₂. These materials represent rare examples of Mo-, Rh-, and Ru-based hybrid porous solids. The synthesis and activation of these cages were optimized to maximize porosity, yielding BET surface areas of up to 832 m²/g. Gas adsorption studies with CO₂ and CO reveal distinctive uptake behaviors linked to the metal cations, with Mo₂₄(bdc)₂₄ demonstrating the highest gravimetric CO₂ uptake (2.12 mmol/g at 298 K) and [Ru₂₄(bdc)₂₄]Cl₁₂ exhibiting the strongest CO binding (−75 kJ/mol). Additionally, we explore the selective adsorption of unsaturated hydrocarbons, such as ethylene and propylene, revealing strong binding interactions at low pressures as a result of strong metal–hydrocarbon interactions based on pi-backbonding interactions.